a) Field of the Invention
The present invention relates to a stepping motor and, in particular, to a stepping motor in which the detent torque is eliminated or reduced while the driving torque is increased.
Here, by the detent torque is meant a torque which, even when the motor is not excited, intends to retain a predetermined position, and it essentially differs from the holding torque generating when a specific phase or phases are excited with the motor electrically turned on, that is, the holding torque represented in terms of stiffness characteristics.
b) Background Prior Art
Conventionally, as this type of stepping motor, there has been available one shown in a cross-sectional view of FIG. 6 which represents the mutual relationship between the stator and the rotor.
FIG. 6 illustrates an example of a two phase stepping motor, in which eight salient poles 1.sub.1, 1.sub.2, 1.sub.3, . . . , 1.sub.7 and 1.sub.8 are disposed at equal pitch angles at the side of a stator 1 while five teeth 3 are each disposed at equal pitches of .tau..sub.s =7.5 degrees on the surface of each stator pole opposed to a rotor 2, and also on the outer circumference of the rotor 2, a plurality of, for example, fifty rotor teeth 4 are each disposed at equal pitch angles of .tau..sub.R =7.2 degrees. Incidentally, reference numeral 5 of FIG. 6 denotes a winding of the stator pole, and 6 an output shaft of the rotor 2.
In FIG. 6, in an attempt to reduce the detent torque of the stepping motor, the pitch .tau..sub.S of the teeth 3 at the side of the stator 1 and the pitch .tau..sub.R of the teeth 4 at the side of the rotor 2 are not set equal.
However, in such a conventional stepping motor, since the cycle of the detent torque of the motor is not systematically considered in determining the pitch .tau..sub.S of the teeth 3 at the side of the stator 1, the detent torque has not been fully reduced.